Auto merge of #14922 - heycam:pseudo-inherit, r=emilio

Don't inherit all properties for eagerly computed pseudos if there are no matching rules.

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When we resolve style for an eagerly computed pseudo-element, such as an anonymous box, if there are no matching rules we currently just clone the parent's style.  We should only do that if `inherit_all` is true, otherwise we should inherit only the inherited properties from the parent.

I was going to use `.unwrap_or_default()` on the result of looking up `precomputed_pseudo_element_decls`, but that didn't seem to work since that map lookup returns a reference and not a `Vec` itself.

r? @emilio

---
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- [X] `./mach build -d` does not report any errors
- [X] `./mach test-tidy` does not report any errors
- [X] These changes fix https://bugzilla.mozilla.org/show_bug.cgi?id=1329121

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- [ ] There are tests for these changes OR
- [ ] These changes do not require tests because _____

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components/script_layout_interface/wrapper_traits.rs

@@ -394,7 +394,7 @@ pub trait ThreadSafeLayoutElement: Clone + Copy + Sized + Debug +
                                     &context.default_computed_values,
                                     false);
                             data.styles_mut().pseudos
-                                .insert(style_pseudo.clone(), new_style.unwrap());
+                                .insert(style_pseudo.clone(), new_style);
                         }
                     }
                     PseudoElementCascadeType::Lazy => {

components/style/stylist.rs

@@ -276,14 +276,18 @@ impl Stylist {
                                          parent: Option<&Arc<ComputedValues>>,
                                          default: &Arc<ComputedValues>,
                                          inherit_all: bool)
-                                         -> Option<ComputedStyle> {
+                                         -> ComputedStyle {
         debug_assert!(SelectorImpl::pseudo_element_cascade_type(pseudo).is_precomputed());
-        if let Some(declarations) = self.precomputed_pseudo_element_decls.get(pseudo) {
+
+        let rule_node = match self.precomputed_pseudo_element_decls.get(pseudo) {
+            Some(declarations) => {
                 // FIXME(emilio): When we've taken rid of the cascade we can just
                 // use into_iter.
-            let rule_node =
                 self.rule_tree.insert_ordered_rules(
-                    declarations.into_iter().map(|a| (a.source.clone(), a.importance)));
+                    declarations.into_iter().map(|a| (a.source.clone(), a.importance)))
+            }
+            None => self.rule_tree.root(),
+        };
 
         let mut flags = CascadeFlags::empty();
         if inherit_all {
@@ -298,10 +302,7 @@ impl Stylist {
                                 None,
                                 Box::new(StdoutErrorReporter),
                                 flags);
-            Some(ComputedStyle::new(rule_node, Arc::new(computed)))
-        } else {
-            parent.map(|p| ComputedStyle::new(self.rule_tree.root(), p.clone()))
-        }
+        ComputedStyle::new(rule_node, Arc::new(computed))
     }
 
     /// Returns the style for an anonymous box of the given type.
@@ -329,7 +330,6 @@ impl Stylist {
             }
         };
         self.precomputed_values_for_pseudo(&pseudo, Some(parent_style), default_style, inherit_all)
-            .expect("style_for_anonymous_box(): No precomputed values for that pseudo!")
             .values
     }
 

ports/geckolib/glue.rs

@@ -521,10 +521,10 @@ pub extern "C" fn Servo_ComputedValues_GetForAnonymousBox(parent_style_or_null:
 
 
     let maybe_parent = ComputedValues::arc_from_borrowed(&parent_style_or_null);
-    let new_computed = data.stylist.precomputed_values_for_pseudo(&pseudo, maybe_parent,
+    data.stylist.precomputed_values_for_pseudo(&pseudo, maybe_parent,
                                                &data.default_computed_values, false)
-                           .map(|styles| styles.values);
-    new_computed.map_or(Strong::null(), |c| c.into_strong())
+        .values
+        .into_strong()
 }
 
 #[no_mangle]